1. Field of the Invention
The present invention relates to an input/output coupling structure between a dielectric waveguide and a printed circuit board on which the dielectric waveguide is to be mounted, and, more particularly, to a bandwidth widening technique for the input/output coupling structure.
2. Description of the Related Art
In recent years, mobile communications devices have become widespread, and frequencies of GHz bands have come to be used for communications therebetween. Then, in a base station which relays the communication between the mobile communications devices, a dielectric waveguide filter has been used which comprises a combination of a plurality of waveguide resonators.
The dielectric waveguide filter can be dramatically downsized due to a wavelength shortening effect of dielectric body as compared to a conventional cavity waveguide, and thereby can be utilized while being directly mounted on a printed circuit board.
However, the dielectric waveguide, and a strip line used in the printed circuit board, are different from each other in terms of a transmission mode of electromagnetic wave. Thus, in the case of using the dielectric waveguide such as the dielectric waveguide filter while being directly mounted on the printed circuit board, it is necessary to provide an input/output coupling structure having a wideband characteristic for converting a mode from the strip line to the dielectric waveguide.
FIG. 5 is an exploded perspective view illustrating an example of a conventional dielectric waveguide input/output coupling structure described in JP 2000-049506A, showing a case of a dielectric waveguide 1 mounted on a surface of a printed circuit board 4. In FIG. 5, a region exposing the dielectric body is slashed.
As illustrated in FIG. 5, the dielectric waveguide 1 comprises an approximately rectangular parallelepiped-shaped dielectric body having an exterior coated with an electrically conductive film.
The dielectric waveguide 1 has a side surface provided with a coupling window 2 exposing the dielectric body, for coupling to other dielectric waveguide which is not illustrated, and a bottom surface provided centrally with an approximately quadrangular shaped electrode 3 surrounded by an electrically conductive film in spaced-apart relation thereto.
The printed circuit board 4 has a front surface-side ground pattern 5 provided on a front surface of a substrate 7, and a back surface-side ground pattern 6 provided on a back surface of the substrate 7.
The printed circuit board 4 has a front surface provided with an approximately quadrangular shaped electrode 5a surrounded by the front surface-side ground pattern 5 in spaced-apart relation thereto, and a back surface provided with a strip line 6a disposed in spaced-apart relation to the back surface-side ground pattern 6.
A center of the electrode 5a and a distal end of the strip line 6a are coupled together by a via hole 7.
The dielectric waveguide 1 is mounted on the printed circuit board 4 in such a manner as to allow the electrode 3 provided on the dielectric waveguide and the electrode 5a provided on the printed circuit board 4 to be opposed to each other.